Ramakrishna Podila and Apparao Rao at the Clemson Nanomaterials Center, along with graduate students Jingyi Zhu and Anthony Childress, have discovered how to increase by five-fold the energy capacity of supercapacitors without sacrificing strength or durability using specially designed layers of atom-thick carbon sheets called graphene.
For the average person who may use but never see a supercapacitor, Clemson’s work means faster charging times, longer lives, a lighter power source than batteries, reduced dependency on fossil fuels, tons less air pollution and possibly lower energy prices.
Graphene sealed in a pouch with electrolytes makes a flexible supercapacitor.
Image Credit: Ashley Jones / Clemson University
In Geneva, Switzerland, supercapacitors power public buses two kilometers from a 15-second charge, and interest in Clemson’s research is building.
“A national research and development enterprise in India is interested in the Clemson supercapacitors and visited the Clemson Nanomaterials Institute twice. Negotiations for manufacturing supercapacitors to power a bus are in progress,” said Rao, the Robert A. Bowen Professor of Physics in the College of Science.
Other potential applications of supercapacitors are far-reaching, from regenerative braking in hybrid and electric vehicles to providing the burst of power needed to adjust the direction of turbine blades in changing wind conditions.
Capacitors, unlike batteries, deliver a lot of power over a very short time. Batteries deliver less power, but they store much more energy. Batteries store energy through a chemical reaction: ions in lithium ion batteries move between negative and positive electrodes.
“While the chemical reactions hold much energy, the ion motion in batteries is rather slow, leading to low power,” said Podila, an assistant professor in physics and astronomy in the College of Science.
Supercapacitors overcome this by storing ions on the surface of nanomaterials electrostatically, like socks sticking to towels coming out of a dryer.
Graphene, the nanomaterial used by the Clemson team, is ultrathin, a million times thinner than a human hair. It’s stronger than steel, flexible and lightweight; a sheet the size of a football field would weigh less than a gram.
“The high-surface area of graphene provides space for ion storage (high-energy) and the ions are always on the surface ready to race (high power),” Podila said. “The problem, however, has been to effectively use the high surface area.”
Often, Podila said, ions can’t access some of the spaces in nanomaterials due to lack of connectivity. Also, the electrons within some nanomaterials may limit the total energy of a supercapacitor through an effect called “quantum capacitance”.
The Clemson team created microscopic layers of graphene with nanometer-sized pores, then sandwiched them together. The pores not only open new channels for ions to access all the spaces in graphene, but they also increase the quantum capacitance.
Creating the pores in specific configurations increased storage capacity 150 percent. Then the researchers introduced two different electrolytes whose ions were smaller than the pores; one by 20 percent, the other by 55 percent.
The effect was like spreading mayo on soft, light, porous bread; the electrolytes oozed into the pores.
“Testing showed the electrolytes with the larger ions did not increase the capacity, but the smaller ions travel through the pores into untapped parts of graphene. The result was a 500 percent increase in capacity,” Zhu said.
Furthermore, the graphene retained its electrical and material properties; the bread, soaked with mayo, didn’t fall apart.
Zhu and Childress also fashioned graphene into thin, flexible electrodes and inserted them into a flexible pouch. They filled the pouch with the electrolyte containing the smaller ions and sealed it, creating a lightweight, flexible supercapacitor that withstood more than 10,000 charge-discharge cycles without any loss in performance.